There are several basic methods for reducing harmonic voltage and current distortion from non-linear distribution loads such as variable speed drives (VSDs) or adjustable frequency drives (AFDs). Following is a description of each method, along with each method’s advantages and disadvantages.
No Method Used
Assuming an AFD with a 6-diode input bridge, and no line reactor, DC choke, or filter applied, IEEE519: 1992 guidelines may not be met even if the total connected AFD loads are less than 10%. Since many AFDs require a minimum 1 — 3% input impedance, the AFD requirements may not be met without a line reactor or additional impedance.
Note: The SVX9000 comes standard with a nominal 3% input impedance.
Advantages
¦ No added cost
¦ Easy to package
¦ Easy to sell
¦ Easy to apply
Disadvantages
¦ Potentially high levels of harmonic current and voltage distortion
¦ AFD is more susceptible to damage caused by line transients
¦ AFD impedance requirements may not be met
DC Choke
This is simply a series inductance on the DC side of the semiconductor bridge circuit on the front end of the AFD. In many ways, the DC choke is comparable to an equivalent AC-side line reactor, although the %Total Harmonic Distortion (THD) is somewhat less. The DC choke provides a greater reduction primarily of the 5th and 7th harmonics. On higher order harmonics the line reactor is superior, so in terms of meeting IEEE guidelines, the DC choke and line reactor are similar. If a DC choke (or line reactor) is applied on all AFDs, it is possible to meet IEEE guidelines where up to 15% to 40% of system loads are AFDs, depending on the
stiffness of the line, the amount of linear loads and the value of choke inductance. A harmonic analysis is required to guarantee compliance with guidelines.
Advantages
¦ Packaged integrally to the AFD
¦ Can provide moderate reduction in voltage and current harmonics
¦ Less voltage drop than an equivalent line reactor
Disadvantages
¦ Less protection than other methods for the AFD input semiconductors
¦ May not reduce harmonic levels to below IEEE519: 1992 guidelines
¦ Impedance is typically fixed by design (not selectable)
¦ Not available as an option for most AFDs, including the SVX9000
Line Reactor
A line reactor is a three-phase series inductance on the line side of an AFD. If a line reactor is applied on all AFDs, it is possible to meet IEEE guidelines where up to 15% to 40% of system loads are AFDs, depending on the stiffness of the line and the value of line reactance. Line reactors are available in various values of percent impedance, most typically 1 — 1.5%, 3% and 5%.
Note: The SVX9000 comes standard with a nominal 3% input impedance.
A harmonic analysis is required to guarantee compliance with guidelines.
Advantages
¦ Low cost
¦ Can provide moderate reduction in voltage and current harmonics
¦ Available in various values of percent impedance
¦ Provides increased input protection for AFD and its semiconductors from line transients
Disadvantages
¦ May require separate mounting or larger AFD enclosure (for SVX9000 if more than 3% is required)
¦ May not reduce harmonic levels to below IEEE519: 1992 guidelines
12-Pulse Converters
A 12-pulse converter incorporates two separate AFD input semiconductor bridges, which are fed from 30° phase shifted power sources with identical impedance. The sources may be two isolation transformers, where one is a delta/wye design (which provides the phase shift) and the second a delta/delta design (which does not phase shift). A line reactor of equal impedance to the delta/wye transformer may also be used in lieu of the delta/delta transformer. The 12-pulse arrangement allows the harmonics from the first converter to cancel the harmonics of the second. Up to approximately 85% reduction of harmonic current and voltage distortion may be achieved (over standard 6-pulse converter). This permits a facility to use a larger percentage of AFD loads under IEEE519: 1992 guidelines than allowable using line reactors or DC chokes. A harmonic analysis is required to guarantee compliance with guidelines.
Advantages
¦ Reasonable cost, although significantly more than reactors or chokes
¦ Substantial reduction (up to approx. 85%) in voltage and current harmonics
¦ Provides increased input protection for AFD and its semiconductors from line transients
Disadvantages
¦ Impedance matching of phase-shifted sources is critical to performance
¦ Transformers often require separate mounting or larger AFD enclosures
¦ May not reduce distribution harmonic levels to below IEEE519: 1992 guidelines
¦ Cannot retrofit for most AFDs
12-Pulse Distribution
This is similar to a 12-pulse converter, on a macro scale. If two AFDs of equal horsepower and load are phase shifted by feeding one AFD from a delta/wye transformer, and feeding the second through a delta/delta transformer or a line reactor of equivalent impedance, performance similar to 12-pulse may be achieved. The cancellation will degrade as the loads vary from AFD to AFD, although as the load on a
single AFD decreases, the individual distortion contribution percentage decreases, resulting in less of a need for cancellation. It is possible for a facility with a large number of AFDs to feed two halves of the distribution from phase-shifted transformers, yielding a large reduction in harmonic levels for minimal cost, and allowing a higher percentage of AFD loads under IEEE519: 1992 guidelines. A harmonic analysis is required to guarantee compliance with guidelines.
Advantages
¦ Cost may either be low or high depending on implementation
¦ Provides substantial reduction (50 — 80%) in voltage and current harmonics
¦ Provides increased input protection for AFD and its semiconductors from line transients
Disadvantages
¦ Cost may be low or high depending on implementation
¦ Impedance and load-matching of phase-shifted sources is critical to performance
¦ Transformers will require separate mounting
¦ May not reduce harmonic levels to below IEEE519: 1992 guidelines
Harmonic Trap Filters
Harmonic trap filters are usually used in conjunction with a line reactor, and are usually placed on individual AFD loads. They are usually an L-C filter installed in a shunt arrangement on the line side of the AFD, and are tuned somewhat below the 5th harmonic, which is the largest component of harmonic distortion. A significant amount of 7th harmonic distortion will also be absorbed. Additional filters tuned to higher order harmonics may also be used. More care is needed with the application of harmonic trap filters than with other methods, since they will tend to try to filter the entire distribution system of harmonic components. If additional AFD or non-linear loads are added without filtering, the previously installed filters may become
overloaded (they are generally fused for protection). The line reactor is used in conjunction with the filter to minimize the possibility of this occurring and to enhance filter performance. A harmonic analysis is required to guarantee compliance with guidelines.
Advantages
¦ Allow a higher percentage of AFD system loads than line reactors and chokes
Disadvantages
¦ High cost
¦ Separate mounting required
¦ May not reduce harmonic levels to below IEEE519: 1992 guidelines
¦ Care is needed in application to ensure that the filter will not become overloaded
¦ Distribution changes, such as adding AFDs, could lead to overloading
Broadband Filters
These filters are similar to trap filters but have some major design differences. As trap filters are connected in parallel to the AFD, broadband filters are connected in series with the AFD and carry the full AFD current. This difference provides added protection for the input power section of the AFD. Broadband filters require no tuning, improve power factor for the system and minimize all harmonic frequencies, including the 3rd harmonic. Additionally, they avoid system resonance and importation of outside harmonics.
Advantages
¦ Allows a higher percentage of AFD system loads than line reactors and chokes
¦ Provides increased input protection for AFD and its semiconductors from line transients
¦ Provides added protection for AFD input power section
¦ Provides system power factor correction
Disadvantages
¦ High cost
¦ Separate mounting required
¦ May not reduce harmonic levels to below IEEE519: 1992 guidelines
¦ Could result in leading power factors at during lightly loaded conditions
¦ Requires modification to match with an AFD using internal line reactors, such as the SVX9000
Eaton’s Clean Power (18-Pulse Converter)
This method is similar to 12-pulse converters, although instead of using two phase-shifted power sources and semiconductor bridges, three are used. Eaton uses a specially wound autotransformer (Differential Delta) and 18-input semiconductors. When this arrangement is used, over 90% of harmonic currents are canceled (typical total harmonic distortion of 2 — 3%).
Advantages
¦ Virtually guarantees compliance with IEEE519: 1992
¦ Provides increased input protection for AFD and its semiconductors from line transients
¦ Up to four times the harmonic reduction of 12-pulse methods
¦ Smaller transformer than isolation transformer used in 12-pulse converter
Disadvantages
¦ Can be more expensive than other methods
¦ Larger and heavier magnetics than some other methods
Active Filters
This method uses sophisticated electronics and power section IGBTs to inject equal and opposite harmonics onto the power system to cancel those generated by other equipment. These filters monitor the non-linear currents demanded from non-linear loads (such as AFDs) and electronically generate currents that match and cancel the destructive harmonic currents. Active filters are inherently non-resonating and are easily connected in parallel with system loads.
Advantages
¦ Guarantees compliance with IEEE519: 1992 if sized correctly
¦ Harmonic cancellation from the 2nd to 51st harmonic
¦ No series connection provides easy installation with no major system rework
¦ Provides VAR currents, improving system power factor
Disadvantages
¦ Can be more expensive than other methods due to high performance control and power sections
¦ The filter’s input semiconductors are exposed to line transients
For assistance in the application of drives or harmonic reduction methods, or for a free harmonic analysis, contact
your local Eaton Electric® office or Drives Solution partner iOpen Pty Ltd.